Light emitting diodes (LEDs) are increasingly used for lighting, but still there are some technological challenges to overcome, in particular with regard to large-scale processing, in order to reach the real breakthrough.
Over recent years the interest in nanowire technology has increased. In comparison with LEDs produced with conventional planar technology nanowire LEDs offer unique properties due to the one-dimensional nature of the nanowires, improved flexibility in materials combinations due to less lattice matching restrictions and opportunities for processing on larger substrates. Suitable methods for growing semiconductor nanowires are known in the art and one basic process is nanowire formation on semiconductor substrates by particle-assisted growth or the so-called VLS (vapor-liquid-solid) mechanism, which is disclosed in e.g. U.S. Pat. No. 7,335,908. Particle-assisted growth can be achieved by use of chemical beam epitaxy (CBE), metalorganic chemical vapour deposition (MOCVD), metalorganic vapour phase epitaxy (MOVPE), molecular beam epitaxy (MBE), laser ablation and thermal evaporation methods. However, nanowire growth is not limited to VLS processes, for example WO 2007/102781 shows that semiconductor nanowires may be grown on semiconductor substrates without the use of a particle as a catalyst. One important breakthrough in this field was that methods for growing group III-V semiconductor nanowires, and others, on Si-substrates have been demonstrated, which is important since it provides a compatibility with existing Si processing and expensive III-V substrates can be replaced by cheaper Si substrates.
One example of a bottom emitting nanowire LED is shown in WO 2010/14032. This nanowire LED comprises an array of semiconductor nanowires grown on a buffer layer of a substrate, such as a GaN buffer layer on a Si substrate. Each nanowire comprises an n-type nanowire core enclosed in a p-type shell and a p-electrode with an active layer formed between the n-type and p-type regions that form a pn or pin junction. The buffer layer has the function of being a template for nanowire growth as well as serving as a current transport layer connecting to the n-type nanowire cores. Further the buffer layer is transparent since the light that is generated in the active area is emitted through the buffer layer.
Although nanowire LEDs have advantageous properties and performance, the processing with regard to contacting of the nanowire LEDs requires new routes as compared to planar technology. Since nanowire LEDs comprise large arrays of nanowires, thereby forming a three-dimensional surface with high aspect ratio structures, deposition of contact material using line-of-sight processes is a challenging operation.